An adhesive tape or sheet for application to skin generally has an adhesive layer formed on at least one surface of a substrate, and is adhered to the target skin surface via the adhesive layer. The adhesives used for an adhesive layer of such adhesive tape or sheet for application to skin are generally classified into rubber adhesives and acrylic adhesives.
A rubber adhesive is a composition containing a rubber component such as natural rubber, isoprene rubber, styrene/isoprene/styrene rubber and the like, and a tackifier, a softener and the like. This composition shows superior adhesion to a relatively dry skin surface. However, since rubber adhesives generally have high hydrophobicity and low moisture permeability, when an adhesive tape or sheet including this adhesive is applied to skin, the skin surface tends to get steamy due to sweat etc. to cause irritation to the skin. In summer, during sports and bathing when a lot of sweat is inevitable, moreover, the adhesion to the skin becomes radically poor.
An acrylic adhesive comprises a (meth)acrylic acid alkyl ester monomer as a main component monomer, and when copolymerized with various monomers, affords an adhesive having desired and well-balanced properties. Because control of the adhesive property is relatively easy, it is widely used as an adhesive layer of an adhesive tape or sheet for application to skin. Conventional acrylic adhesives relatively easily impart high hydrophilicity and high moisture permeability to an adhesive layer, thereby preventing steamy skin surfaces, but are inferior to rubber adhesives in adhesion to the skin. Conventional acrylic adhesives, like rubber adhesives, rapidly lose skin adhesiveness when exposed to much sweat in summer, during sports, bathing and the like.
Thus, the adhesive tapes or sheets for application to skin now in the market show superior adhesion to relatively dry skin surfaces, but once the skin surfaces get sweaty, cannot maintain the adhesiveness or sufficient adhesion to the skin.
Accordingly, there is a demand on an adhesive tape or sheet for application to skin that causes less steaminess of the skin surface and can show superior adhesion to the skin during perspiration.
An adhesive tape or sheet for application to skin may be required to secure adhesion to the skin surface for a long time, in addition to the capability of not causing steamy skin surface and adhesion during perspiration. To meet such requirements, the acrylic adhesive is crosslinked, gel content of the adhesive is carefully controlled, and skin adhesiveness and internal cohesion are balanced. When the gel content is too high, the internal cohesion becomes too high and the adhesive power to the skin decreases, as a result of which the adhesive tape or sheet for application to skin tends to become loose from the edge or come off easily.
An improved adhesion to the skin means a higher risk of intensifying irritation to the skin by peeling off of an adhesive from the skin. For the irritation upon peeling of an adhesive to be decreased, a liquid or paste of a plasticized component is added to the adhesive, and the adhesive is crosslinked to maintain the balance between high skin adhesive power and internal cohesion, thereby reducing the skin irritation.
In an attempt to solve the above-mentioned problems, there have been proposed a method including crosslinking an acrylic adhesive by ionization irradiation, thereby increasing the internal cohesion of an adhesive layer (JP-A-11-226109), and a method including addition of a liquid or paste of a plasticized component to an acrylic adhesive and crosslinking the acrylic adhesive by ionization irradiation, thereby decreasing skin irritation (U.S. Pat. No. 5,543,151). However, there is a risk that direct ionizing radiation of an acrylic adhesive may cause direct bonding of polymer chains of acrylic copolymer in the adhesive, which in turn causes too high an internal cohesion and intolerably low adhesion to the skin. When a plasticized component is contained in the acrylic adhesive, direct ionizing radiation for crosslinking makes it difficult to retain plasticized components between polymer chains, with the result of exudated plasticized component from the adhesive layer. In this event, the irritation to the skin cannot be decreased, and exudated plasticized component contaminates the skin, thereby possibly inducing even stronger irritation to the skin.
Thus, an adhesive tape or sheet for application to skin, which causes less steaminess of the skin surface, can show superior adhesion to the skin for a long time during perspiration, and which causes less irritation to the skin has been also demanded.